Overlooking what is underground: Root:shoot ratios and coarse root allometric equations for tropical forests

Coarse roots are a major belowground carbon (C) pool, yet we know little about the factors controlling coarse root distributions in tropical forests, the biome where belowground biomass stocks are largest. We conducted a literature survey to identify potential environmental controls on the root:shoot biomass ratio (R:S) across tropical forests worldwide. Moreover, to aid efforts to extrapolate aboveground biomass measurements to belowground C stocks, we also compiled a list of allometric equations for coarse root biomass. Across our large and geographically comprehensive dataset, we found that R:S is negatively correlated with mean annual precipitation, and positively linked with stand age. However, information on root biomass allocation is still scarce in comparison with the wealth of available data on aboveground plant biomass in tropical forests, and more data will be necessary to understand the drivers of belowground biomass allocation. Moreover, the available root allometric equations show tremendous variation in mathematical form, which is likely related to the many different methods that have been used to quantify root biomass and its distribution with depth. A better understanding of belowground dynamics in tropical forests will require systematic surveys of root biomass along gradients of soil type, climate, and stand age, along with a streamlined protocol for developing root allometric equations.